Method of forming embedded copper interconnections and embedded copper interconnection structure

ABSTRACT

Embedded interconnections of copper are formed by forming insulating layer, forming an embedded interconnections of copper in the insulating layer, making an exposed upper surface of the insulating layer and an exposed surface of the embedded interconnections of copper coplanar according to chemical mechanical polishing, and forming a protective silver film on the exposed surface of the embedded interconnections of copper. These steps are repeated on the existing insulating layer thereby to produce multiple layers of embedded interconnections of copper. The exposed surface of the embedded interconnections of copper is plated with silver according to immersion plating.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of forming embeddedinterconnections of copper on a surface of a substrate such as asemiconductor wafer, and a structure of such embedded interconnectionsof copper.

2. Description of the Related Art

Generally, aluminum alloys have heretofore been used as the materials ofinterconnections for use in semiconductor devices. For lower electricresistance and greater migration resistance, however, embeddedinterconnections of copper produced by a damascene process, and suchembedded interconnections of copper arranged in multiple layers areemployed.

Conventional multilayer embedded interconnections of copper havesuffered various problems. Such problems will be described below withreference to FIGS. 1A through 1C of the accompanying drawings whichillustrate a process of successive steps of forming multilayer embeddedinterconnections of copper.

As shown in FIG. 1A, an interconnection 111 of copper is embedded in theupper surface of an insulating layer 110 of silicon dioxide (SiO₂).Another insulating layer 120 of silicon dioxide (SiO₂) is disposed onthe insulating layer 110 and the interconnections 111 for insulating theinterconnections of copper (Cu) 111 in an upper layer. When theinsulating layer 120 is deposited on the interconnections layer 111, anexposed upper surface 111a of the interconnections 111 is undesirablyoxidized by oxygen.

As shown in FIG. 1B, an etchant (etching gas) is applied to etch theinsulating layer 120 through a hole 131 defined in a resist layerpattern 130 on the surface of the insulating layer 120 for therebyforming a hole 121 in the insulating layer 120, which will be filledwith a plug for connection to the interconnections 111. When theinsulating layer 120 is thus etched, the exposed upper surface 111a ofthe interconnections 111 is undesirably modified in the composition bythe etchant.

As shown in FIG. 1C, when the resist layer 130 (see FIG. 1B) is removedusing oxygen, the exposed upper surface 111a of the interconnections 111is undesirably oxidized by the applied oxygen.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a methodof forming embedded interconnections of copper while effectivelypreventing an exposed surface of an the interconnections of copper frombeing modified or oxidized.

Another object of the present invention is to provide an embedded copperinterconnection structure.

According to the present invention, a method of forming embeddedinterconnections of copper/comprises the steps of: forming an insulatinglayer; forming embedded interconnections of copper in the insulatinglayer; planarizing (making coplanar) an exposed surface of theinsulating layer including an exposed surface of the embeddedinterconnections of copper; and forming a protective film of silver onthe exposed surface of the embedded interconnections of copper. Theprotective film of silver on the embedded interconnections of copperprevents the embedded interconnections of copper from being oxidized andprevents the surface thereof from being modified.

To form the protective film of silver on the exposed surface of theembedded interconnections of copper, the exposed surface of the embeddedinterconnections of copper may be plated with silver according toimmersion plating. The immersion plating allows the protective film ofsilver to be selectively formed only on the exposed surface of theembedded interconnections of copper, and also allows the protective filmof silver to be formed in a very small thickness. Therefore, the amountof silver in the protective silver film may be relatively small. Sincethe protective film of silver and the embedded interconnections ofcopper do not form a solid solution, the electrical resistance of theembedded interconnections remains relatively low even if silver andcopper are diffused in each other.

According to the present invention, an embedded copper interconnectionstructure comprises a substrate, a first insulating layer disposed onthe substrate, an embedded interconnection of copper disposed in thefirst insulating layer, a protective film of silver disposed on theembedded interconnections of copper in the first insulating layer. Thestructure also includes a second insulating layer disposed on a surfaceof the first insulating layer including the embedded interconnections ofcopper having the protective film of silver thereon.

The above and other objects, features, and advantages of the presentinvention will become apparent from the following description when takenin conjunction with the accompanying drawings which illustrate apreferred embodiment of the present invention by way of example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A through 1C are fragmentary cross-sectional views showing aconventional process of successive steps of forming multilayer embeddedinterconnections of copper;

FIG. 2 is a perspective view illustrative of a process of forming aprotective film of silver (Ag) on interconnections of copper disposed inthe surface of an insulating layer of SiO₂ according to the presentinvention;

FIG. 3 is a diagram showing the relationship between the composition ofa eutectic alloy of copper and silver and the resistivity at a certaintemperature;

FIGS. 4A through 4D are fragmentary cross-sectional views showing themanner in which the protective film operates.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As shown in FIG. 2, interconnections 15 of copper are formed by platingso as to be embedded in the surface of an insulating layer 10 of SiO₂ onthe surface of a semiconductor wafer (or substrate) 25. The entire uppersurface of the insulating layer 10 including the exposed upper surfaceof the interconnections 15 is made planar by chemical mechanicalpolishing (CMP) (the upper surface being defined as t he surface facingaway from the substrate). The semiconductor wafer is then dipped in anaqueous solution of silver cyanide to plate only the exposed surface ofthe interconnections 15 with a thin protective film 17 of silver (Ag)according to immersion plating. The protective film 17 is shownexaggerated as having an appreciably large thickness in FIG. 2, althoughit has a very small actual thickness on the exposed upper surface of theinterconnections 15.

The immersion plating for forming the protective film 17 is carried outaccording to the following formula: ##EQU1##

The silver cyanide reacts only with the copper of the interconnections15, but not with the silicon dioxide of the insulating layer 10.Therefore, during the immersion plating, the protective film 17 isselectively formed only on the exposed upper surface of theinterconnections 15.

The immersion plating allows the protective film 17 to be formed in avery small thickness, and may require only a relatively small amount ofsilver for the protective film 17.

FIG. 3 shows the relationship between compositions of a eutectic alloyof copper and silver, which do not form a solid solution, and theresistivity at a certain temperature T₀. It can be seen from FIG. 3 thatif the concentration of copper in the eutectic alloy is close to 100%(point ρ_(B)), then the resistance of the eutectic alloy issubstantially the same small value as if the concentration of copper is100%. Therefore, since the protective film 17 is very thin and containsa relatively small amount of silver, the overall resistance of theinterconnections 15 remains essentially unchanged even if silver andcopper diffuse in each other.

As shown in FIG. 4A, a second insulating layer 20 of SiO₂ is formed onthe first insulating layer 10, including the first layer ofinterconnections 15 and the protective film 17, for supportinginterconnections of copper in an upper layer. At this time, theprotective film 17 prevents the interconnections 15 from being oxidized.

As shown in FIG. 4B, a resist layer 30 is formed on the upper surface ofthe insulating layer 20. Then an etchant of fluorine gas is applied tothe insulating layer 20 through a hole 31 defined in the resist layer 30for forming a hole 21 in the insulating layer 20, which will be filledwith a plug 15 for connection to the interconnections 15. At this time,the etchant contacts the protective film 17, but not theinterconnections 15. Therefore, the interconnection 15 is prevented frombeing modified by the etchant.

The resist layer 30 shown in FIG. 4B is then oxidized and removed byashing, as shown in FIG. 4C. At this time, the protective film 17prevents the interconnections 15 from being oxidized.

After a plug 35 is formed in the hole 21 in the insulating layer 20 anda second layer of interconnections 36 is embedded in the insulatinglayer 20, the overall surface of the insulating layer 20 is planarizedby chemical mechanical polishing. If another insulating layer of SiO₂ isto be formed on the planarized insulating layer 20, then a protectivelayer of Ag 37 may be formed on the exposed upper surface of the nextinterconnections 36 by immersion plating. Thus, as shown in FIG. 4D, oneembodiment of the present invention includes a plurality of insulatinglayers 10, 20, formed on a substrate 25. Each of the insulating layers10, 20 has embedded interconnections of copper 15, 36. A protective filmof silver 17, 37, is formed on each of the embedded interconnections ofcopper.

In the above embodiment, the overall upper surface of each of theinsulating layers is planarized by chemical mechanical polishing.However, they may be planarized by any of various planarizing processesother than chemical mechanical polishing.

While the aqueous solution of silver cyanide is employed in theimmersion plating process in the above embodiment, another solution suchas an aqueous solution of silver nitrate may be employed in immersionplating.

The present invention offers the following advantages. The protectivefilm of Ag on the exposed upper surface of the interconnections of Cuprevents the interconnections from being oxidized or modified.Therefore, the yield of the semiconductor devices will be improved, andthe production throughput of semiconductor wafers will be increased.

Because the protective film of Ag is formed on the exposed upper surfaceof the interconnections of Cu by immersion plating, the protective filmof Ag can be selectively formed only on the exposed surface of theinterconnections of Cu without a mask. The protective film can have avery small thickness, which will reduce the amount of Ag that diffusesin the Cu interconnections. Hence, the electrical resistance of theinterconnections will be prevented from increasing.

Although a certain preferred embodiment of the present invention hasbeen shown and described in detail, it should be understood that variouschanges and modifications may be made therein without departing from thescope of the appended claims.

What is claimed is:
 1. An embedded copper interconnection structurecomprising:a substrate having an upper surface; an insulating layerdisposed on said substrate so as to cover substantially all of saidupper surface of said substrate, said insulating layer having embeddedcopper interconnections and an upper surface, each of said embeddedcopper interconnections having an exposed upper surface; and aprotective silver film formed on said exposed upper surface of each ofsaid embedded copper interconnections by immersion plating.
 2. Thecopper interconnection structure of claim 1, wherein said upper surfaceof said insulating layer and said exposed upper surface of each of saidembedded copper interconnections are formed to be coplanar by chemicalmechanical polishing.
 3. The copper interconnection structure of claim1, wherein said protective silver film is formed after said chemicalmechanical polishing.
 4. The copper interconnection structure of claim1, wherein said protective silver film has a small thickness and is freeof a solid solution having copper.
 5. The copper interconnectionstructure of claim 1, wherein said insulating layer comprises a firstinsulating layer and said embedded copper interconnections comprisefirst embedded copper interconnections, further comprising a secondinsulating layer disposed on said first insulating layer and said firstembedded copper interconnections, said second insulating layer havingsecond embedded copper interconnections, said second insulating layerhaving holes for establishing connections between said first embeddedcopper interconnections and said second embedded copperinterconnections.
 6. A multilayer embedded copper interconnectionstructure comprising:a substrate having an upper surface; a plurality ofinsulating layers disposed in succession on said substrate, saidplurality of insulating layers including a first insulating layerdisposed on said substrate so as to cover substantially all of saidupper surface of said substrate, each of said insulating layers havingembedded copper interconnections and an upper surface, each of saidembedded copper interconnections having an exposed upper surface; and aprotective silver film formed on said exposed upper surface of each ofsaid respective embedded copper interconnections by immersion plating.7. The multilayer copper interconnection structure of claim 6, whereinsaid upper surface of each of said plurality of insulating layers andsaid exposed upper surface of each of said respective embedded copperinterconnections are formed to be coplanar by chemical mechanicalpolishing.
 8. The multilayer copper interconnection structure of claim6, wherein said protective silver film is formed after said chemicalmechanical polishing.
 9. The multilayer copper interconnection structureof claim 6, wherein said protective silver film has a small thicknessand is free of a solid solution having copper.